Nanostructures include so-called one-dimensional nanoelements, essentially in one-dimensional form, that are of nanometer dimensions in their width and diameter, and are commonly known as nanowires, nanowhiskers, nanorods, nanotubes, etc. For the purpose of this application the term nanoelement is used. As regards e.g. nanowires and nanowhiskers, the basic process of nanostructure formation on substrates by the so-called VLS (vapor-liquid-solid) mechanism is well known. However, the present invention is limited to neither the nanowires nor the VLS process, e.g. selectively grown nanowires and nanostructures, etched structures, other nanoelements, and structures fabricated from nanowires are also included, although the description is focused on VLS-grown nanowires. Methods for growing nanowires on semiconductor substrates are described in US Published Applications 2003/010244 and 2004/0075464.
Semiconductor nanoelement devices show great promise, potentially outperforming standard electrical, opto-electrical, and sensor—etc. semiconductor devices. These devices can utilize certain nanoelement specific properties, 2D, 1D, or 0D quantum confinement, flexibility in axial material variation due to less lattice match restrictions, antenna properties, ballistic transport, wave guiding properties etc. Furthermore, in order to design first rate semiconductor devices from nanoelements, i.e. transistors, light emitting diodes, semiconductor lasers, and sensors, and to fabricate efficient contacts, particularly with low access resistance, to such devices, the ability to dope and fabricate doped regions is crucial. The general importance of doping is easily exemplified by the basic pn-junction, a structure being a critical part of several device families, where the built in voltage is established by doping of the p and n regions.
As an example the limitations in the commonly used planar technology are related to difficulties in making field effect transistors, FET, with low access resistance, the difficulty to control the threshold voltage in the post-growth process, the presence of short-channel effects as the planar gate length is reduced, and the lack of suitable substrate and lattice-matched heterostructure material for the narrow band gap technologies.
One advantage of a nanoelement FET is the possibility to tailor the band structure along the transport channel using segments of different band gap and or doping levels. This allows for a reduction in both the source-to-gate and gate-to-drain access resistance. These segments may be incorporated directly during the growth, which is not possible in the planar technologies.
The doping of nanoelements is challenged by several factors. Physical incorporation of dopants into the nanoelement crystal may be inhibited, but also the established carrier concentration from a certain dopant concentration may be lowered as compared to the corresponding doped bulk semiconductor. One factor that limits the physical incorporation and solubility of dopants in nanoelements is that the nanoelement growth temperatures very often are moderate. US25006673 teaches a method of providing charge carriers to a nanowhisker from adjacent layers.
For VLS grown nanoelements, the solubility and diffusion of dopant in the catalytic particle will influence the dopant incorporation. One related effect, with similar long term consequences, is the out-diffusion of dopants in the nanoelement to surface sites. Though not limited to VLS grown nanoelements, it is enhanced by the high surface to volume ratio of the nanoelement. Also, the efficiency of the doping—the amount of majority charge carriers established by ionization of donors/acceptor atoms at a certain temperature may be lowered compared to the bulk semiconductor, caused by an increase in donor or acceptor effective activation energy, due to the small dimensions of the nanoelement. Surface depletion effects, decreasing the volume of the carrier reservoir, will also be increased due to the high surface to volume ratio of the nanoelement.
The above described effects are not intended to establish a complete list, and the magnitudes of these effects vary with nanoelement material, dopant, and nanoelement dimensions. They may all be strong enough to severely decrease device performance.